Mi Ri Joung

Piezoelectric nanogenerators synthesized using KNbO3 nanowires with various crystal structures

KNbO3 (KN) nanowires having a tetragonal structure or a polymorphic phase boundary (PPB) structure, which contains both tetragonal (P4mm) and orthorhombic (Amm2) structures, are formed at low temperatures. The presence of tetragonal and PPB KN nanowires is attributed to the existence of OH− and H2O defects. Further, the tetragonal and PPB KN nanowires change to orthorhombic KN nanowires in the temperature range between 300 and 400 °C owing to desorption of the lattice hydroxyl group. A composite consisting of polydimethylsiloxane (PDMS) and KN nanowires having a PPB structure shows large dielectric constant and low dielectric loss values of 9.2 and 0.5%, respectively, at 100 kHz. Moreover, a nanogenerator (NG) synthesized using the PPB KN nanowires exhibits the largest output voltage and current among NGs synthesized using the tetragonal or orthorhombic KN nanowires. In particular, the NG containing 0.7 g of PPB KN nanowires shows an output voltage of 10.5 V and an output current of 1.3 μA; these values are among the highest ever reported for NGs synthesized using a lead-free composite. In addition, this NG exhibited the maximum output power and energy conversion efficiency, which were 4.5 μW and 0.9%, respectively, for an external load of 1.0 MΩ.

Bipolar Resistive Switching Behavior of a Pt/NiO/TiN Device for Nonvolatile Memory Applications

Bipolar resistive switching behavior was observed in a Pt/NiO/TiN device. The device exhibited switching behavior that was stable over 100 cycles and did not degrade after 104 s. An electroforming process was required to obtain these bipolar resistive switching properties, and the conduction behavior of the low resistance state followed Ohms law, indicating that conductive filaments formed during the electroforming process. The conductive filaments consisted of oxygen vacancies and the Pt electrode behaved as an oxygen reservoir. The bipolar resistive switching of the Pt/NiO/TiN device was explained by the generation and annihilation of oxygen vacancies in the filaments.

Structural variation of hydrothermally synthesized KNbO3 nanowires

KNbO3 (KN) nanowires were synthesized using various process conditions and their structures and morphologies were investigated. Homogeneous KN nanowires were formed in specimens synthesized at 130 °C for 24.0–48.0 h. These KN nanowires have a tetragonal structure that is known to be stable at high temperatures in the range of 225–435 °C. Tetragonal KN nanowires changed to orthorhombic KN nanoplates when the process time increased and homogeneous orthorhombic KN nanoplates existed for specimens synthesized for 144.0 h. In addition, tetragonal and orthorhombic structures coexisted in KN nanoplates synthesized at 130 °C for 72.0 h. For specimens synthesized at 100 °C, a long process time of 144.0 h was required to develop homogeneous KN nanowires that were also considered to have both tetragonal and orthorhombic structures. On the other hand, for specimens synthesized at 150 °C for 8.0 h, KN nanowires and a cube-shaped KN phase coexisted. Furthermore, a K4Nb6O17 second phase was formed in specimens synthesized for short periods of time (<8.0 h), indicating that the formation of homogeneous KN nanowires is difficult at 150 °C. Therefore, homogeneous KN nanowires with a tetragonal structure can be obtained at a low temperature of 130 °C with a short process time in the range of 24.0–48.0 h.KNbO3 (KN) nanowires were synthesized using various process conditions and their structures and morphologies were investigated. Homogeneous KN nanowires were formed in specimens synthesized at 130 °C for 24.0–48.0 h. These KN nanowires have a tetragonal structure that is known to be stable at high temperatures in the range of 225–435 °C. Tetragonal KN nanowires changed to orthorhombic KN nanoplates when the process time increased and homogeneous orthorhombic KN nanoplates existed for specimens synthesized for 144.0 h. In addition, tetragonal and orthorhombic structures coexisted in KN nanoplates synthesized at 130 °C for 72.0 h. For specimens synthesized at 100 °C, a long process time of 144.0 h was required to develop homogeneous KN nanowires that were also considered to have both tetragonal and orthorhombic structures. On the other hand, for specimens synthesized at 150 °C for 8.0 h, KN nanowires and a cube-shaped KN phase coexisted. Furthermore, a K4Nb6O17 second phase was formed in specimens synthesiz...

Synthesis of highly tetragonal BaTiO3 nanopowders using acetone as a solvent by alkoxide-hydroxide route

BaTiO3 (BT) nanopowders were synthesized in acetone to remove the TiO2 second phase, which was formed in the specimens synthesized in the 2-methoxyethanol solvent at above 100 °C due to H2O evaporation. No TiO2 second phase was formed in the specimens synthesized in acetone, even at 260 °C, due to the low boiling temperature of acetone which evaporated at above 55 °C and suppressed the H2O evaporation by increasing the vapor pressure in the vessel. However, for the BT nanopowders synthesized at 250 °C for 60 h, TiO2 second phase was formed at a Ba/Ti ratio of ≤1.5 and the BT nanopowders were agglomerated at a ratio of ≥3.0. Homogeneous BT nanopowders were obtained at a ratio of 2.0 and they exhibited a high c/a ratio of 1.0082 with a small size of 95.2 nm. Therefore, these nanopowders can be a good candidate for future multi-layer ceramic capacitor.